The overall objective of the project is to determine the influence of systemic injection of adrenergic, dopaminergic, cholinergic and serotoninergic agonists and antagonists on the basal secretion and the afternoon surge of prolactin. Further, selective agonists and antagonists will be microinjected into the ventromedial-arcuate and preoptic regions of the hypothalamus and their influence on the basal and on the afternoon surge of prolactin will be studied. The animal model to accomplish these objectives will be estrogenized, ovariectomized female rats with indwelling aortic catheters. Results to date indicate that both alpha and beta adrenergic antagonists increase the basal levels of serum prolactin but the intensity and the duration of stimulation differ between drugs and dosages administered. Alpha adrenergic agonists but not beta agonists also increase prolactin secretion but it is of short lived duration. Dopamine receptor stimulators decrease while dopamine receptor blockers inhibit the basal secretion of prolactin. Cholinergic blockers and stimulators have little effect on basal serum prolactin levels except for a nicotinic cholinergic receptor blocker, mecamylamine, which gives a transient increase. Both serotonin receptor blockers and stimulators increase prolactin secretion. Further studies indicate that methiothepin, a serotonin receptor blocker, also has a potent action on blocking dopamine receptors.